Pallet dispensers are used in warehouses and the like to dispense empty reusable pallets for use as required. One class of dispensers uses sensor means to determine whether a pallet is present or not, and if no pallet is in the dispensed position, to dispense a pallet. An example of this type of apparatus is disclosed in the co-pending commonly assigned U.S. application Ser. No. 842,328 now abandoned. This disclosure uses whisker switches to sense a pallet. However, whisker switches are subjected to considerable force in a typical pallet dispenser environment, and are prone to damage, particularly when a pallet is moved so as to reverse the direction of travel and so force the whisker directly into the pallet.
Conventionally, a light and photoelectric sensor arrangement is used, such that anything blocking a clear sight causes the dispenser to refrain from dispensing a further pallet. This type of arrangement is known from, for example, U.S. Pat. No. 4,557,656 to Ouetlette, and Australian application Ser. No. 40895/78 to U.E.B. Industries Ltd. The blockage is intended to be a pallet, however in an industrial situation, tape, cardboard, or other rubbish may obscure clear sight and prevent a further pallet being dispensed.
U.S. Pat. Nos. 4,047,622 to Blessing. 3,623,618 to Shaw, and 4,172,686 to Shorthouse disclose mechanical arrangements which sense the height of pallets, or the associated mechanism, rather than the presence or absence of a pallet at the dispensed position or positions.
It is accordingly an object of the present invention to provide an improved pallet sensing arrangement.